Unless otherwise indicated, the foregoing is not admitted to be prior art to the claims recited herein and should not be construed as such.
Power amplifiers are commonly used in various applications such as speaker drivers, headphone amplifiers, telephone line drivers, etc. There are several classes of power amplifiers, including for example, Class A, Class, B, and Class D amplifiers.
In a Class A amplifier, the output devices are continuously conducting for the entire cycle, or in other words there is always bias current flowing in the output devices. This topology has the least distortion and is the most linear, but at the same time is the least efficient because of the continuous operation of the amplifier.
In a Class B amplifier, the output devices only conduct for half the sinusoidal cycle (one conducts in the positive region, and one conducts in the negative region). If there is no input signal, then there is no current flow in the output devices. Class B amplifiers operate more efficiently than Class A amplifiers, but at the cost linearity at the crossover point between conduction and non-conduction.
A Class D amplifier is a switching or PWM amplifier. In this kind of amplifier, the switches are either fully on or fully off, significantly reducing the power losses in the output devices. The audio signal is used to modulate a PWM carrier signal which drives the output devices, with the last stage being a low pass filter to remove the high frequency PWM carrier frequency.
Portable electronic devices (smart phones, MP3 players, computer tablets, etc.) typically use miniature speakers, which tend not to be very robust and can fail due to overheating. An important consideration with high-end Class D amplifiers, therefore, is the ability to measure the load impedance in order to protect the speaker from being damaged.